The present invention relates to valve train systems for use on internal combustion engines; more particularly, to devices for controllably varying the lift and/or timing of valves in such engines; and most particularly, to means for controlling the lash between the camshaft lobes and a two-arm rocker arm in a valve train system having cam lobes for both opening and closing an individual valve.
Devices for controllably varying the degree of lift and the timing of opening and closing valves in internal combustion engines are well known. See, for example, U.S. Pat. No. 5,937,809 issued Aug. 17, 1999 to Pierik et al., and U.S. Pat. No. 6,019,076 issued Feb. 1, 2000 to Pierik et al., the relevant disclosures of which are herein incorporated by reference. Such a device is referred to in the art as a Variable Valve Mechanism (VVM). Such devices commonly employ a rocker arm which pivots with or about a shaft or pin as a part of the apparatus train. Typically, the rocker arm has a first bearing element, for example a roller, which follows the profile of a cam lobe during rotation of a camshaft.
Conventional variable valve mechanisms typically include many component parts, such as link arms, joints, pins, and return springs, and are thus relatively complex mechanically. Return springs are used typically to maintain the roller in contact with the input cam lobe and to reduce mechanical lash as the input cam lobe rotates from a high lift position to a low lift position. The use of such return springs negatively impacts the durability of the VVM and also may limit the operating range of the mechanisms, thereby limiting the operation of the intake valve throttle control system to a correspondingly-limited range of engine operation.
It is known to provide a second cam lobe per valve in place of return springs, and to employ a two-armed rocker arm sub-assembly having appendages in contact with both the opening lobe and the closing lobe at all times. The angular orientation between the eccentrics of the opening and closing lobes on the camshaft defines the rotational angle through which the valve is open. Typically, the surface of the opening lobe is followed by a roller mounted on the first rocker arm, and the surface of the closing lobe is followed by a slider mounted on the second rocker arm. Such an arrangement provides positive control of the rocker arm sub-assembly, and thus of the associated valve, at all positions of the camshaft and obviates the need for return springs.
A practical problem can arise in manufacturing and assembling such a two-lobe system. The stack-up of machining and mounting tolerances among the rocker, the roller, the pivot shaft for the rocker, the two cam lobes, and the camshaft mounting in the engine head can be formidable. Ideally, the roller and slider are just lightly in contact with the base circles of their respective cam lobes during the non-lift portions of the rotational cycle. If this lash relationship is too tight, i.e., zero or negative clearance, the valve may not open or close properly, or the rocker arm assembly may be stressed and distorted. If the lash relationship is too loose, the rocker arm assembly may clatter or chatter undesirably against the cam lobes, and the valve may not open fully or precisely in time.
What is needed is a simple means whereby the valve train components may be manufactured and assembled with loose tolerances and then the lash relationship of the cam followers to the cam lobes may be easily and precisely adjusted and retained after the valve train is assembled.
It is a principal object of the present invention to provide improved apparatus and method for setting the lash relationship of cam followers to cam lobes in a two-cam, two-follower valve train.
It is a further object of the invention to provide such a system wherein the setting may be conveniently and accurately done after the valve train is assembled.
Briefly described, a rocker arm assembly in accordance with the invention has a first arm for following a first or opening camshaft lobe and a second arm for following a second or closing camshaft lobe, the arms being designated with respect to a pivot axis of the rocker arm therebetween. Preferably, the first arm is provided with a captive roller follower and the second arm is provided with a captive sliding follower or slider. These elements are so selected for economy because opening of the valve is more mechanically demanding than is closing it. The pivot axis of the rocker arm, the surface of the roller at the contact point with the opening lobe, and the surface of the slider at the contact point with the closing lobe, taken together define a triangle in space which must fit exactly into the space requirements of the valve train assembly of each valve in a multi-cylinder engine. In accordance with the invention, the shape of the triangle is mechanically and controllably adjustable to change the location of the slider with respect to the other two points of the triangle and to the camshaft axis of rotation, to adjustably control the mechanical lash in the system.